In an image forming apparatus in an electrostatic copying system capable of multicolor image formation, such as a color copier or a color printer, image formation is performed by forming toner images of respective colors on latent image carriers such as photoreceptor drums, transferring these toner images to an intermediate transfer belt sequentially to form a multicolor image, and then transferring and fixing the multicolor image to a recording paper.
The recording paper used for image formation has various quality and forms. As a recording paper generally used for a printer and a copier, a recording paper supplied by a manufacturer of the copier or the like is used, whereas in developing countries and the like, for example, an inexpensive recording paper used in that country is used. Some of recording paper is worse and has surface asperity, and also its size, quality and the like are different from a standard recording paper recommended by the manufacturer.
In addition, an embossed paper subjected to embossing finish intentionally on the surface of the paper so as to have subtle irregularity such as a pear skin like pattern, a cloth like pattern, or a silk like pattern is known. Alternatively, there is also a recording paper that is used in a form of being folded in two or in three. For example, a card board used for an invitation to a wedding, an event or the like is shipped in a state of being folded in two during papermaking, and it becomes necessary to perform printing on such a paper folded in two. Even in the case of the recording paper in a state of being folded once in this manner, irregularity is partially caused on the surface thereof.
When printing is performed for the recording paper having various quality and forms as described above, performing image formation processing by an electrophotographic system as described above allows a large quantity of printing processing at a low price. However, in the case of the recording paper whose surface is worse or having a large irregularity by embossing, for example, there is a problem that the intermediate transfer belt is not able to sufficiently contact with the whole surface of the recording paper due to irregularity of the surface, thus lowering a transfer ratio of a toner and causing deterioration of image quality.
In addition, when a paper folded in two is printed, surface contact between a transfer member and the recording paper becomes uneven in the vicinity of a folding line of the recording paper, so that there is a problem that excellent transfer is not able to be performed in the vicinity of the folding line, thus causing deterioration of image quality.
As to a printing technology for addressing such paper quality and forms, for example, Japanese Laid-Open Patent Publication No. 4-315162 discloses an image forming apparatus aiming to obtain a clear image excellent in fixing property of a toner with respect to many kinds of papers for image formation. In this image forming apparatus, surface smoothness of a paper is measured by a paper smoothness measuring portion, and when the smoothness is lower than a predetermined level, an image forming operation is not performed and the paper is passed being sandwiched between a heat roller and a pressure roller for smoothing, and then guided to a path for duplex copy, followed by image formation.
When the above image formation by an electrostatic copying system is performed for a recording paper whose surface roughness is rough (large degree of irregularity) as described above, it is difficult to cause the surface of the recording paper to contact with the smooth intermediate transfer belt uniformly and stably, thus posing a problem that image quality is deteriorated significantly depending on the state of the recording paper.
Against these problems, such a measure is taken that, for example, an adhesion amount of a toner for performing image formation is increased or transfer current is increased to facilitate transfer of a multicolor image on the intermediate transfer belt to a recording paper. However, problems are caused that when the adhesion amount of a toner is increased, an image such as a character becomes in the swollen state on the recording paper to deteriorate image quality, and when the transfer current is increased, retransfer is caused due to overcurrent transfer to deteriorate image quality. Accordingly, merely by adjusting image forming conditions, it is impossible to address recording papers with various surface states sufficiently.
Moreover, in the case of a recording paper with a folding line, printing is performed in a state where the recording paper that is shipped with a folding line is unfolded. For example, a recording paper shown in FIG. 15A is in a state of being folded in three, which needs to be made in an open state as shown in FIG. 15B when being printed. In this case, a trace of the folding line is not able to be eliminated sufficiently only by smoothing out with a hand of a user, so that the folding line part is floated from the intermediate transfer belt to cause transfer failure.
For example, when an outward of a folding line is caused to contact with the intermediate transfer belt, the vicinity of the outward of the folding line is floated from the intermediate transfer belt to be separated therefrom. In addition, when printing the inward of the folding line, the vicinity of the inward of the folding line becomes impossible to contact with the intermediate transfer belt. Even when image formation is performed with the folding line opened in this manner, image deterioration is caused due to transfer failure at the folding line part or in the vicinity of the folding line. In particular, a stiff card board is often used for the recording paper for an invitation to an event or the like, and the folding line part is not able to be smoothed, so that deterioration of image quality is caused easily. Accordingly, such a design is conventionally made that image formation is not performed in the vicinity of the folding line part.
In contrast, as shown in Japanese Laid-Open Patent Publication No. 4-315162, in accordance with smoothness of a recording paper subjected to image formation, by passing through a fixing portion without performing image formation to pressure and heat the recording paper by the fixing portion, it is possible to smooth the recording paper to some extent.
However, in the case of smoothing the recording paper by passing through the fixing portion, for example, when a card board is passed through the fixing portion plural times, the card board has a peculiarity to be curled so that a problem is caused that a jam rate is increased in a conveyance path. In addition, when a thin paper is passed through the fixing portion, a problem is caused that pressure force of the fixing portion is too large to generate wrinkles on the thin paper to cause failure. That is, even when smoothing the recording paper by the fixing portion, it is necessary to perform fixing under the condition corresponding to characteristics of the recording paper. A technical idea about such optimization is not disclosed in Japanese Laid-Open Patent Publication No. 4-315162.